1. Field of the Invention
The present invention relates to a linear photoelectric array with optical gain. More particularly, the present invention relates to a infrared linear photoconductive optical array using plural low-cost Lead salt (PbSe) detectors or image elements with a linear optical gain element increasing both the detectivity and responsivity of the array.
2. Discussion of the Related Technology
Linear photoelectric arrays are used for a variety of purposes. For example, such arrays may be used in optical measurement devices. In such a use the image or shadow of an object being measured is projected onto the array, and the number of image elements of the array which are illuminated by the image or darkened by the shadow is used as an indication of the physical size of the object. Also, such linear arrays are useful for imaging in which the array is swept across an image field, or in which a scanner provides a line-by-line series of linear image fragments to the array. The photoelectric response of the array converts these linear image fragments into a series of electrical signals which can be used to construct a complete image of the entire scene.
Such linear photoelectric arrays may also be used to image visible or invisible light. For example, such an array may be used to image infrared radiation. Conventional linear arrays which are used to detect infrared radiation generally use detector elements of InSb, or HgCdTe, which are of comparatively high cost. Additionally, these arrays may require that they be cooled to temperatures which are significantly below ambient in order to image infrared radiation with a sufficient level of responsivity and detectivity. These detectors might be cooled to a temperature of 77.degree. K., for example.
The low temperatures required with conventional infrared detectors may require cryogenic fluids or chillers to be used to cool the array, as well as the use of specialized insulation techniques including such precautions as shielding the array from radiant heating. Of course, such cooling and insulation precautions further increase the expense of using such conventional detectors in addition to their already high cost.
Conventionally, rather large unitary (as opposed to array) infrared detectors of Lead Sulfide or Lead Selenide have been provided with optical gain units in the form of hemispherical lens of Silicon, Germanium, or Strontium Titinate. These unitary detectors were not of the small-element type, but were generally of a size about 1 mm square. Such hemispherical concentrating lenses were associated with the infrared detector by the use of an optical cement. The use of the cement, particularly with respect to its thickness, had to be carefully controlled because the cement can cause optical degradation by refractive index mismatch, by defining internal voids which caused light scattering, and by defining reflective interfaces which also cause light scattering, for example. These conventional optical gain units in the form of a hemispherical concentrating lens generally provided a gain factor of about 1.5. They were of no use to increase the detectivity and responsivity of small-element linear array infrared detectors.
The responsivity of an infrared detector is a measure of the output signal level provided by the detector in comparison to the radiated power of the image source with which the detector is associated. The usual unit for responsivity is amps/watt. Similarly, the detectivity of an infrared detector is an indication of the ability of the detector to respond to the long wave length radiations of the infrared portion of the spectrum, also in comparison with the radiated power of the source. The unit of measurement for detectivity is cm. Hz.sup.1/2 /watt.